The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11b-1999 Supplement to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, and more commonly referred to as “802.11b” or “WiFi”, has become the darling of the information technology industry and computer enthusiasts alike as a wired LAN/WAN alternative because of its potential 11 Mbps effective throughput, ease of installation and use, and transceiver component costs make it a real and convenient alternative to wired 10 Base T Ethernet and other cabled data networking alternatives. With 802.11b, workgroup-sized networks can now be deployed in a building in minutes, a campus in days instead of weeks since the demanding task of pulling cable and wiring existing structures is eliminated. Moreover, 802.11b compliant wireless networking equipment is backwards compatible with the earlier 802.11 1M/2 Mbps throughput standard, thereby further reducing deployment costs in legacy wireless systems.
802.11b achieves relatively high payload data transmission rates or effective throughput via the use of orthogonal class modulation in general, and, more particularly, 8-chip complementary code keying (“CCK”) and a 11 MHz chipping rate to bear the payload. As such, previously whitened or scrambled bitstream data of interest is mapped into nearly orthogonal sequences (or CCK code symbols) to be transmitted, where each chip of the CCK code symbol is quaternary phase modulated using QPSK (“quadrature phase shift keying”) modulation techniques. Meanwhile the common phase of each CCK symbol is jointly determined by the current and previous symbols using differential QPSK or DQPSK modulation scheme. Subsequent conversion into the analog domain prepares these CCK symbols for delivery over a wireless medium RF modulated on a carrier frequency within the internationally recognized 2.4 GHz ISM band to form the payload or PLCP Service Data Unit of an 802.11b compliant Physical Layer Convergence Procedure (“PLCP”) frame, a type of packet. The high-rate physical layer PLCP preamble and header portions forming the frame overhead are still modulated using the 802.11 compliant Barker spreading sequence at an 11 MHz chipping rate. In particular, the preamble (long format—144 bits, short format—72 bits) is universally modulated using DBPSK (“differential binary phase shift keying”) modulation resulting in a 1 Mbps effective throughput, while the header portion may be modulated using either DBPSK (long preamble format) or DQPSK (short preamble format) to achieve a 2 Mbps effective throughput.
An IEEE 802.11b compliant receiver receives and downconverts an incident inbound RF signal to recover an analog baseband signal bearing the PLCP frame, and then digitizes and despreads this signal to recover the constituent PLCP preamble, header and payload portions in sequence. The preamble and header portions are Barker correlated and then either DBPSK or DQPSK demodulated based on the preamble format used to recover synchronization data and definitional information concerning the received PLCP frame, including the data rate (Signal field in the PLCP header) and octet length (Length field in the PLCP header) of the variable-length payload or PSDU portion. The CCK encoded symbols forming the PLCP payload portion are each correlated against 64 candidate waveforms in received per symbol sequence in combination with DQPSK demodulation to verify and reverse map each into the underlying bitstream data of interest, at either 4 bits per symbol (5.5 Mbps) or 8 bits per symbol (11 Mbps) increments.
It should be appreciated that 802.11 and 802.11b signals operate in the 2.4 GHz ISM band and must therefore coexist with quite an array of dissimilar signals operating in the same frequency, including microwave ovens and digital phones. By definition, there are no licensure restrictions within the available RF channels of the ISM band, so 802.11 and 802.11b compliant transceivers must employ clear channel assessment techniques to determine if it is safe to transmit. In particular, there is an expected amount of ambient noise that the 802.11/802.11b transceivers must tolerate but still be able to transmit, but should not attempt to transmit while another in-range 802.11/802.11b transceiver is transmitting so as to maximize channel use and system throughput. In other words, it is desirable for 802.11 and 802.11b transceivers to know when the operating channel is occupied with valid traffic, and thus enter receive mode without attempting to transmit over such traffic. Likewise, it is desirable that these transceivers should be free to transmit on the operating channel while that channel is free of 802.11/802.11b traffic, even in the presence of a tolerable amount of noise or interference.
To this end, the 802.11 and 802.11b standards specify clear channel assessment (CCA) guidelines which are used to determine if a tuned RF channel contains valid PLCP frame traffic. Inbound signals in the tuned or operating RF channel which do not meet these CCA guidelines are considered to bear either corrupted frames, or represent interference or noise in the channel. The 802.11/802.11b CCA guidelines are organized in modes as follows:                CCA Mode 1: Energy above threshold. CCA shall report a busy medium upon detecting any received energy above the ED threshold.        CCA Mode 2: Carrier Sense only. CCA shall report a busy medium only upon detection of a DSSS signal. This signal may be above or below the ED threshold.        CCA Mode 3: Carrier Sense with energy above threshold. CCA shall report a busy medium upon detection of a DSSS signal with energy above the ED threshold.        CCA Mode 4 (802.11b): Carrier sense with timer. CCA shall start a timer whose duration is 3.65 ms and report a busy medium upon the detection of a High Rate PHY signal. CCA shall report an IDLE medium after the timer expires and no High Rate PHY signal is detected. The 3.65 ms timeout is the duration of the longest possible 5.5 Mbps PSDU.        CCA Mode 5 (802.11b): A combination of carrier sense and energy above threshold. CCA shall report busy at least while a High Rate PPDU with energy above the ED threshold is being received at the antenna.The 802.11 DSSS PHY receiver must perform CCA according to at least one of modes 1-3, and the 802.11b High Rate PHY must perform CCA according to modes 1, 4 or 5.        
Three of the five conventional CCA modes require thresholding inbound signal energy, and so this guideline is believed important. However, conventional transceivers simply compare inbound signal energy against the specified threshold, and report an energy threshold validation signal whenever the threshold is exceeded. Thus, the presence of strong interference in the operating channel, will cause (in the case of a CCA mode 1 implementation) or potentially may (in the case of a CCA mode 3 or 5) cause a false busy to be reported, and thus prevent the transceiver from transmitting, which may in turn cause transmission delay and lower effective data throughput.
Moreover, to implement CCA modes 2-5, conventional CCA carrier sense techniques are used to determine if a DSSS or High Rate PHY inbound signal is present, typically by thresholding a measure of the perceived Barker code lock. However, known techniques are relatively complex and are thus power inefficient and expensive to implement. Both cost and power consumption reduction are critical design goals in 802.11/802.11b transceiver implementation, it would be advantageous if simpler carrier sense techniques could be incorporated without materially affecting carrier sense sensitivity or recognition performance.
Further, while conventional CCA techniques look for valid PLCP header information (via CRC validation), there is no post-demodulation confirmation during receipt of the preamble. Checking for valid preamble receipt would be advantageous, especially where the inbound signal fades potentially below the inbound signal energy threshold, but the receiver is still able to successfully recover recognizable preamble information from the signal.
Finally, while the defined 802.11/802.11b CCA modes account accommodate a range of operational environments, they are not appropriate for every environment and channel condition. Therefore, it would advantageous to provide a transceiver capable of handling further CCA modes other than those defined by the 802.11/802.11b standards, preferably while retaining backwards compatibility with such standards.